In recent years, due to the increasing diversification of information technology equipment there is an increasing need for surface light emitting devices with low power consumption and small volume, and electroluminescent devices (hereinafter abbreviated as EL devices) which are one such surface light emitting devices are attracting a lot of attention.
Such EL devices are broadly classified into inorganic EL devices and organic EL devices depending on types of material used in them.
Further, in case of inorganic EL devices, generally a high electric field is applied to the light emitting section, and electrons collide with light emission centers being accelerated by that high electric field, thus the light emission centers are excited and cause the emission of light. On the other hand, in case of organic EL devices, electrons and holes are injected respectively from the electron injection electrode and hole injection electrode into the light emission layer, the organic material get into an excited state, and the emission of light is caused when this organic material returns from the excited state to the ground state. And organic EL devices have the advantage that they are capable of being driven at a lower voltage than the inorganic EL devices.
Further, in the case of organic EL devices, it is possible to obtain light emitting devices that emit light with the appropriate color tone by selecting appropriate light emitting materials, it is also possible to obtain white light by suitably combining the light emitting materials, and hence they can be expected to be used as the back light for liquid crystal display devices, etc.
Further, when used as the back light for liquid crystal display devices, etc., usually a front luminosity of 2000 to 4000 cd/m2 is required. But when light is emitted using a surface light emitting device such as the above EL device, etc., the emitted light travels in all directions, and a lot of light is totally reflected at the light emitting side surface of the surface light emitting device and is kept in the interior of the device and hence it is difficult to obtain sufficient front luminosity. And particularly in the case of organic EL devices, there was the problem that in order to obtain sufficient light emission life it was only possible to obtain a front luminosity of about 1000 to 1500 cd/m2.
Further, conventionally, when light is emitted from a surface light emitting device such as an organic EL device, in order to improve its front luminosity, proposals have been made to provide minute undulations in the light emitting side surface of the surface light emitting device (see, for example, Patent Document 1), or to the affix a flat member provided with undulations on the light emitting side surface of the surface light emitting device so that these undulations appear on the surface (see, for example, Patent Document 2).
However, if, in the above manner, minute undulations are provided on the light emitting side surface of surface light emitting devices, or if a flat member provided with undulations is affixed on the light emitting side surface of the surface light emitting device so that these undulations appear on the surface, there was the problem that light got dispersed due to the undulations on the surface and it was not yet possible to sufficiently increase the front luminosity.
Patent Document 1: Japanese Unexamined Patent Application Open to Public Inspection No. 9-63767
Patent Document 2: Japanese Unexamined Patent Application Open to Public Inspection No. 5-45505